t~olo c{).34-

15
01&6_ c{).34- 26. Draina ge co nditions at the Fores tr y Commi ss ion nurs ery, Pe rth . D.J . Sloane The Forestry Commission requested advice on possible solutions to drain - age problems at thei " r pine seedling nursery at Perth [EPl S1969]. They thought that disease in the young pine seedlings was partly a result of poor drain - age conditions resulting in a high water table and surface ponding of water after periods of heavy rain . The nursery covers about 6 ha and is situated about 500 m north of the Midland Highway bridge over the South Esk River . The land is gently undul- ating but drops steeply to the South Esk River to the west. GENERAL GEOLOGY The geology of the Forestry Commission nursery-Native Point area la r gely consists of Tertiary sediments associated with a minor amount of Tertiary basalt which crops out on the river bank to the south of the nursery . Fern- tree Mudstone of Permian age crops out in a small area of the river bank , west of Native Point . A thin veneer of Quaternary river gravels and sands, and aeolian sands, mantles most of the area . METHODS OF INVESTIGATION During preliminary investigations seventeen hand-augered holes were bored to obtain an indication of subsurface conditions. A blanket of wind - blown sand of variable thickness was found to overlie a heavy grey and brown mottled clay, presumably of Tertiary age. Subsequently some 200 auger holes were bored. These holes were bored on a grid system with a 15 m interval in a N- S direction and a 20 m interval in an E- W direction . The grid was pegged prior to drilling the auger holes and the position and reduced level of each peg was subsequently surveyed. This enabled a ground surface contour map of the nursery to be drawn (fig. 86) . The purpose of augering the grid system of holes was to determine the subsurface conditions as accurately as possible . Once the conditions were known, the planning of subsequent drainage could be conducted efficiently . In seve r al areas the complete profile from the ground surface to the heavy clay could be determined, however where the aeolian sand was r elatively thick , augering of the saturated sand below the water table was impossible . Augering was conducted using a tractor - mounted post - hole digger to a depth of about one metre . Hand augers were used below this depth . The depth below the ground surface of each soil horizon, the depth to the wa ter table and an estimation of the rate of water seepage , were recorded for each hole . From the auger hole information, sections have been drawn along lines A, C, E, G, I, K, M, and O. These sections (fig. 81 , 82) show the variable th ickness of the sand overlying the clay and the possible existence of a sand filled depression in the underlying clay in the central section of the nur - se ry. In order to obtain an i ndication of the thickness of windblown sand in the cen t ra l area of the nurse ry, two resistivity spreads were carried out , one be t ween Holes A2 and A4 and one between Holes 110 and 112. Between Holes A2 and A4 the clay was b elieved to be close to the ground surface and to ex - te nd to some depth . The use of this spread was to act as a control for the second . Resistivi ty spreads do not produce an accurate result as far as dep t hs to subsurface horizons are concerned , especially if only a few spreads are used, howeve r t he method dOes al low these depths to be estimated . Using a W enner electrode con f iguration the penetration factor is equal to the 226 ,

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T~OlO_ 01&6_ c{).34-26. Drainage conditions at the Fores try Commi ss ion nursery , Perth .

D.J . Sloane

The Forestry Commission requested advice on possible solutions to drain­age problems at thei"r pine seedling nursery at Perth [EPl S1969]. They thought that disease in the young pine seedlings was partly a result of poor drain­age conditions resulting in a high water table and surface ponding of water after periods of heavy rain .

The nursery covers about 6 ha and is situated about 500 m north of the Midland Highway bridge over the South Esk River . The land is gently undul­ating but drops steeply to the South Esk River to the west.

GENERAL GEOLOGY

The geology of the Forestry Commission nursery-Native Point area lar gely consists of Tertiary sediments associated with a minor amount of Tertiary basalt which crops out on the river bank to the south of the nursery . Fern­tree Mudstone of Permian age crops out in a small area of the river bank , west of Native Point . A thin veneer of Quaternary river gravels and sands, and aeolian sands, mantles most of the area .

METHODS OF INVESTIGATION

During preliminary investigations seventeen hand-augered holes were bored to obtain an indication of subsurface conditions. A blanket of wind­blown sand of variable thickness was found to overlie a heavy grey and brown mottled clay, presumably of Tertiary age. Subsequently some 200 auger holes were bored. These holes were bored on a grid system with a 15 m interval in a N- S direction and a 20 m interval in an E- W direction . The grid was pegged prior to drilling the auger holes and the position and reduced level of each peg was subsequently surveyed. This enabled a ground surface contour map of the nursery to be drawn (fig. 86) . The purpose of augering the grid system of holes was to determine the subsurface conditions as accurately as possible . Once the conditions were known, the planning of subsequent drainage could be conducted efficiently . In sever al areas the complete profile from the ground surface to the heavy clay could be determined, however where the aeolian sand was r elatively thick , augering of the saturated sand below the water table was impossible . Augering was conducted using a tractor- mounted post- hole digger to a depth of about one metre . Hand augers were used below this depth . The depth below the ground surface of each soil horizon, the depth to the wa ter table and an estimation of the rate of water seepage , were recorded for each hole .

From the auger hole information, sections have been drawn along lines A, C, E, G, I, K, M, and O. These sections (fig. 81 , 82) show the variable thi c kness of the sand overlying the clay and the possible existence of a sand filled depression i n the underlying clay in the central section of the nur­ser y .

In order to obtain an i ndication of the thickness of windblown sand in the cent ral area of the nurser y , two resistivity spreads were carried out , one be t ween Holes A2 and A4 and one between Holes 110 and 112. Between Holes A2 and A4 the clay was believed to be close to the ground surface and to ex­tend to some depth . The use of this spread was to act as a control for the second . Resistivi t y spreads do not produce an accurate result as far as dept hs to subsurface horizons are concerned , especially if only a few spreads a r e used, however t he method dOes a l low these depths to be estimated . Using a Wenner electrode conf iguration the penetration factor is equal to the

226

,

electrode spacing for uniform unstratified material. Thus for Spread 1 (be­tween AZ and A4) the depth to the base of the clay can be determined with reasonable accuracy at about 6-7 m below the ground surface. From the curve obtained for Spread 1, (fig. 83) the clay is overlying a material with higher resistivity. This material could be bedrock, possibly basalt which crops out along the river bank adjacent to the nursery. The second curve shows the same shape as the first but has material of higher resistance, aeolian sand, overlying the clay. From this curve the sand appears to extend to an approximate depth of 3- 4 m below the ground surface.

GEOMORPHOLOGY

At the nursery , the sediment overlying the heavy clay is aeolian sand of variable thickness, and belongs to the Panshanger.Series of Graley (1961). The sediment has been deposited as a source-bordering sand sheet, probably filling a small depression in the underlying Tertiary clay. The sand sheet was probably deposited during the Last Glacial Stage when aeolian deposition and erosion was associated with more open vegetation conditions and prevail­ing westerly winds. The sediment was associated with the deflation of the alluvial deposits of the South Esk River. Subsequent leaching of the aeolian sands has resulted in a downward movement of clay particles, forming a clay enriched horizon above the underlying Tertiary clay.

A typical soil profile from the ground surface to the underlying clay consists of three horizons (fig. 84). The uppermost horizon is a dark brown, organically enriched, sand generally about 0.5 m in thickness but thicker where artificially increased (e.g. northern end of line A). Below this is a dull yellow-brown to dull yellow-orange horizon of sand in which muscovite mica flakes are readily seen. This is probably the leached AZ horizon of the soil developed on the aeolian sand. Below this is a horizon of dull yellow­brown to light br own mottled clayey sand, probably a clay enriched B2 hori­zon. This overlies a heavy clay which is mottled yellowish brown to greyish yellow- brown and which contains a small percentage of sand- and granule-sized particles. This clay is probably of Tertiary age. Occasionally, small iron­stone concretions are present at the top of the clay-enriched sand horizon.

GRAIN SIZE ANALYSIS

Fifty-two samples were obtained by the Forestry Commission at 26 loc­alities (fig. 89) and analysed at tne laboratory of the Government Analyst (Appendix 1, table 1). Unfortunately, results were given as percentages of sand, silt and clay and no particle size ranges could be obtained. Forty­four of these samples were uniform in grain size distribution and contained 89-93% of sand and 7-11% of silt and clay. On visual inspection eight samples could be distinguished from the remainder. These samples contained from 62.5-87.5% of sand and 12.5-37.5% of silt and clay. Unfortunately, sample cover­age is fairly sparse and only tentative conclusions can be drawn from these results. The samples of higher silt and clay content in the topsoil are at locations 4, 5 , 8, J and K, (K may be disregarded as it is an artificially built seed bed). Samples 4, 8 and J are from areas of poor surface drainage, indicating that in these regions the silt and clay content of the top soil horizon is higher than elsewhere.

Three independent samples were also taken and submitted to the Depart­ment of Mines Laboratory , Launceston, for grain size analysis (Appendix 1, table 2). Samples Sl and S2 were taken from an area where the aeolian sand cover is thick, at depths of 0.1 m and 0.5 m respectively. The grain size distribution for Sl and 52 is very similar to that obtained by Graley (1961) from analysis of similar soils in the Cressy and Longford areas. The grain

227

FORESTRY AUGER HOLE NUMBERS

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Line C

Line E

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FIGURE 81

SECTIONS COMMISSION NURSERY - PERTH

8

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9

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5 cm

REFERENCE

V::'::::;:.·.I A2 horizon of soil profile .. . . ' .. '.' .' leached aeolian sand

:'~ 82 horizon of soil profile ',' mottled clayey sand

~$:ij1tra Mottled cloy, containing .... . ci minor sand

~ Water saturated horizon

- - --- Water tabla

o MaMimum hole dopth

- - - - - - Ironstone concretions

5cale in metres

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45 30 15

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1

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FIGU RE 82

SECTIONS FORESTRY COMMISSION NURSERY PERTH

AUGER HOLE NUMBERS

Line I ---_._-------- - - -

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Line K

2 3 4 5

Line M

2 3 4 5

Line 0

Approx imate depth to cloy deduced from resistiv ity­spread

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Scale in metres

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horizontal

REFERENCE

A1 horizon of soil profile organically enriched

A2 horizon of soil profile leached aeolian sand

82 hor izon of soiL prof ile mottled clayey sand

Mottled clay, contain ing minor sand

Iaf~~ Water saturated hor izon

- - --- Water table

o Maximum hole depth

Scm

o

verticaL

Tech.Rep.Dep.Mines Tasm. 20.

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APPARENT RESISTIVIT Y (!l.m) 10 100

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BEDROCK ~ 1\ BEDROCK

POSS ibl Y j a y llt ('\ Po.'sibly B,a,salt SPREAD 2. (I" -I .. )

~ P~EAD 1 (A ,-A; ) ~ Q

Figure 83. Resistivity curves, Perth Nursery .

,

1000

./

,if

I

Good lateral and vertical drainage

No lateral or vertical drainage

Depth below ground surface (metres)

v V V

V V V V V

Scm

Al horizon - organically enriched - sanoy dark. brown.

A2 horizon(?) aeolian sand containing muscovite mica flakes - dull yellow orange to dull yellow brown.

Water movement occurring along this interface ~n many instances.

82(?) horizon - clay enriched aeolian sand, mottled - dull yellow brown to light brown.

Heavy clay - minor sand content . Mottled­yellowish brown to greyish yellow brown.

Solid bedrock - possibly Tertiary basalt.

N.B. Colours are based on the Revised Standard Soil Colour Charts (OYAMA and TAKEHARA, 1967).

Fi gure 84. Common soil profile, Perth Nursery.

229

size distribution of samples 51 and S2 (fig. 85) shows that the aeolian sand is well sorted and has a median grain size diameter of about 0.2 mm. The sands are positively skewed , that is they show a ' tail ' towards the fine end of the grain size distribution curve , a common feature of aeolian sands . Sample 53 was taken at a depth of 0.1 m ir. the north-eastern corner of the nursery. This clearly shows different grain size characteristics from 51 and 52. It has a smaller median grain size diameter (0 . 11 mm) and a much larger percentage of particles in the smaller grain size (silt and clay) region.

HYDROLOGY

The water table lies above the clayey sand horizon (B2) which contains little free water. There is virtually no vertical or lateral water movement through this horizon which acts as an impermeable barrier to water movement and gives rise to a perched water table. This condition is especially notice­able in areas where the clayey sand horizon is within about a metre of the ground surface. In this case water movement can be clearly seen to be occurr­ing along the A2-B2 interface. The overlying yellow-brown aeolian sand all­ows free vertical and lateral movement of the groundwater. Where the water movement is most rapid (fig . 87) the groundwater gradient is lowest and the cover of aeolian sand is thickest.

Surface and subsurface drainage of most of the nursery area apart from the north-east corner and the area near the southern end of Lines G and I, appears to be good. The water table ranges between 0 .7-1.5 m and averages one metre below the groundsurface . Lateral and vertical subsurface water movement appears to be good.

The main area of poor surface drainage is in the north-eastern corner of the nursery, approximately bounded by Line F to the west and Line 7 to the south . In this region, and also near Lines G and I (S), surface pending of water occurs in small depressions and tractor furrows . This surface ponding is related to several factors . Firstly the texture of the organically enrich­ed Al horizon of the soil profile in this region differs from that of the remainder of the nursery. The sand content is much lower and the clay part­icle content is much higher. This reduces the infiltration capacity of the soil, and combined with the very shallow depth to the clay, does not allow ready vertical and lateral water movement . Clay and organic skins lining small depressions also reflect the poor drainage of the soil and also contrib­ute to it. The direction of cultivation also adds to the problem as it is aligned virtually across the slope. This does not aid surface runoff in the area as tractor furrows tend to act as small dams, adding to the surface drain­age problem. The method of cultivation in this region is also likely to in­fluence the drainage . There are seven beds, with six rows of seedlings to each bed, between the irrigation lines. Each bed is just wide enough to all­ow a trac tor to pass over it with its wheels on either side. Consequently there is a small strip on either side of each bed that is constant ly compacted by the tractor. The method of root-pruning of the pine seedlings effectively loosens the material in each seedling bed. The roots of the seedlings are cut about 100-150 mm below the ground surface by a piano-wire cutter, dragged behind the tractor. The overall effect of cultivation is to loosen shallow ' troughs' of soil and compact narrow strips between . This, combined with the direction of cultivation, will tend to prevent natural downslope movement of water.

A water sample (Appendix 2) was taken from a drain , draining an area between Holes 116-117 and J16-J17. An old well was situated in this region . The water issuing from this area is of good quality, both for human consumpt­ion and irrigation purposes.

230

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Figure 85. Grainsize analysis , Perth Nursery.

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I Median grain size diameter:

51 and 52 = 0'2mm 53 = O·11mm

0·01

SEEDLING GROWTH

A rough sketch map (fig.8S) has been prepared of poor growth and colour areas in the pine seedlings at the Perth nursery . This map has been based on colour prints (scale ~1: 3240) , supplied by the Forestry Commission . These photogr aphs have been interpreted by the Commission and zoned from 1 to 5, Zone 1 being good colour and growth and Zone 5 being bare ground. For con­venience , Zones 1 and 2 have been grouped as good growth and colour areas and Zones 3 to 5 as poor growth and colour areas . In addition 35 rom infra­red transparencies were compared to confirm the general zonings . Photo in­formation was sketched onto the plan of the nursery in the hope that compar­isons bet ween the sketch map and the geological and hydrological information obtained might show some correlation. Unfortunately , no such correlation could be made . Poor growth and colour areas occurred even where the ground is well drained and the sand is thickest. Thus , as thought previously by the Forestry Commission , other factors such as fungi and sand-blasting of the young seedlings during dry conditions are contributing to the disease and poor growth of the seedlings.

CONCLUSIONS AND RECOMMENDATIONS

A ground surface and water table map (fig . 86) has been produced, with a contour interval of 0 . 5 m.

On the basis of a grid system of some 200 auger holes N- S sections have been prepared showing the Subsurface geology and water table (fig. 81 , 82) .

A map indicating, in a qualitative sense, the rapidity of groundwater movement through the aeolian san~s has been prepared (fig .87). This indi­cates the gradient of the water table and essentially confirms the contoured water table map.

From the above information , the laying of drains should be able to be planned more effectively and efficiently.

A sketch map (fig . 88) has been produced showing poor growth and colour areas in young pine seedlings as at 1 July 1975. This map could not be cor­related with the subsurface geology or groundwater conditions.

The conditions giving rise to surface retention of water in the north­eastern corner of the nursery , have been determined. Advice regarding meth­ods of drainage or possible soil restructuring should be obtained from an expert in this field . Surface runoff should be hastened in this region and it is recommended that the irrigation lines and direction of cultivation be reoriented downslope instead of across the slope as at present . A drain should also be dug along the north- eastern boundary . This drain should be dug to the top of the clay and should ensure that any water draining downslope does not enter the nursery .

A drain could also be placed along the northern boundary of the nursery. This should improve the drainage in the north- western corner, allowing water to move more rapidly downslope in this region. The drain will also act as a major drain into which subsequent feeder drains may be discharged.

REFERENCES

GRALEY , A.M. 1961 . The laboratory examination of soils of sheet 47 - Long­ford, Tasmania . Div.Rep.Div . Soils C.S.I . R.O. Aust. 2/61.

NICOLLS , K.D. 1958 . Reconnaissance soil map of Tasmania. Sheet 47 - Lo ng ford. Div.Rep.Div.Soils C.S . I . R.O. Aust. 14/57.

232 {4 Novembe r 1975]

APPENDIX 1 Grai n size analyses.

Table 1 . SAND, SILT AND CLAY CONTENT OF SOIL SAMPLES

SalTple Sand Silt Clay (') (') (,)

IT 91.0 6.0 3 . 0 lB 93.0 3.5 3 . 5 2T 90.5 6 . 0 3.5 2B 90.5 5.5 4 . 0 3T 91.0 5 . 0 4.0 3B 91. 5 5 . 5 3 . 0 4T 83.5 11. 0 5 . 5 4B 91.0 5.5 3 . 5 5T 82.0 13.5 4 . 5 5B 92.0 6 . 0 2 . 0 6T 91.5 5 . 5 3 . 0 6B 92 . 5 3.0 4.5 7T 92 . 5 5 . 0 2 . 5 7B 93 . 0 4.0 3.0 8T 87.5 9.0 3.5 8B 86.5 9.5 4.0 9T 91. 5 4 . 5 4.0 9B 92 . 0 3 . 5 4.5

lOT 91. 5 4.0 4.5 l OB 93 . 0 4.0 3.0 llT 91. 5 4.5 4.0 llB 93 . 0 2.5 4.5 12T 89 . 5 7.5 3.0 12B 90.0 6 . 0 4.0 13T 91. 5 4.0 4.5 13B 90.5 3.0 6 . 5 AT 89 . 0 6.5 4.5 AS 91. 5 4.5 4.0 BT 92 . 0 5.0 3.0 BB 92 . 5 3.5 4.0 CT 91.5 5 . 5 3.0 CB 91. 5 6.0 2.5 DT 91. 5 4.5 4.0 DB 91. 5 4.5 4 . 0 ET 90.5 6 . 5 3.0 EB 90.0 6.5 3.5 FT 91. 0 6 . 0 3.0 FB 92 . 5 4.5 3.0 GI' 90.0 4.5 5.5 GB 92 . 5 3.0 4.5 HT 90.0 5.5 4 . 5 HB 89.5 6 . 0 4.5 IT 92 . 0 4.5 3.5 IB 93.0 4.5 2.5 JT 82 . 0 10 . 0 8.0 JB 62 . 5 9 . 5 28.0 KT 77.0 14.5 8.5 KB 77.0 10 . 5 12.5 LT 91. 5 5.5 3.0 LB 90 . 5 6.0 3 . 5 ><r 89.0 7.5 3.5 MIl 89.5 6 . 0 4.5

Analyses by Government Analyst Laboratory 233

Table 2. GRAIN SIZE DISTRIBUTION OF SOIL SAMPLES 51-53 .

Reg. No . Fraction % Mass Cum. % Mass

751658 (51) +2.36 nun 0.6 0.6* +1.18 mm 0.6 1. 2* +600 vm 0 . 7 1.9* +300 vm 12.2 14.1* +212 vm 31. 3 45.4* +150 vm 23 . 5 68.9* +75 vm 18.4 87.3* +38 ).1m 3.9 91. 2* - 38 J,lm 8.8 100.0

751659 (52) +1.18 mm Trace Trace +600 vm 0.2 0.2 +300 vm 12.1 12.3 +212 ~m 29.2 41. 5 +150 ~m 27.3 68.8 +75 ~m 21. 8 90.6 +38 ).1m 3.2 93.8 - 38 ).1m 6.2 100.0

751660 (53) +2.36 mm 0.4 0.4* +1.18 Iml 0.2 0 . 6* +600 ~m 0.4 1. 0* +300 j.lm 4.6 5.6* +150 J,lm 28.1 33.7* +75 ~m 32.1 65.8* +38 ~m 15.4 81.2* -38 ~m 18.8 100.0

*Fractions contain wood fragments

APPENDIX 2

Analysis of a water sample

The sample, Reg. No. 751657 , was taken from a drain, draining an area between Holes 116-117 and J16-J17. The following analysis was made by the Department of Mines Laboratory, Launceston.

ppm

pH 6 .4 Mg 42

ppm Fe <0.1

C03 0 Al <0.2

HC03 35 K 2.5

Cl 55 Na 30

SO, 67 T.O.S . 500

5i~ 15 Hardness (as CaC03) 260

Ca 32 Alkalinity (as CaC03) 29

234

FIGURE 86

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5 2 3 4 WATER MOVEMENT A

9 10 SCALE : 0 20 ' Om GEOLOGIST : D. J . SLOANE 4 5

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DEPT OF MINES - TASMANIA SEpT 1975 DRAWN : H. MACKINNON 40~O-47 FIGURE 87 Tech.Rep.Dep.Mines Tasm. 20.

DEP~ OF MINES -TASMANIA SEP~ 1975

FIGURE 88

PERTH PINE SEEDLING PLANTATION

SKETCH MAP OF POOR GROWTH & COLOUR IN AREAS OF PINE SEEDLINGS.

SCALE

AS AT 1/7/75

GEOLOGIST : D. J. SLOANE

APPROX. POOR GROWTH & COLOUR AREAS

E%f::1f1 NO SEEDLINGS

DATA : COLOU R 35mm & INFRA RED FORESTRY COMM. PHOTO GRAPHS.

• Approx. mag. N

5cm

DRAWN : H. MACKINNON 4028-47 ~_~.07S4- Tech.Rep.Dep.Mines Tasm. 20.

FIGURE 89

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5CALE o 2 3 4 cnns. rl -.~~--~+I~-,~'.~~~.! o ~ 80 m

MAGNE TIC NORTH (APPROX.) • ,

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FORESTRV COMMISSION SOIL SAMPLE LOCATIOt-S 1-13 : P.rmonent Plots A- M: 13 Roving Plots

MINES DEPARTMENT SOIL SAMPLES 51, 52 & 53

5 cm

DEPARTMENT OF MINES

Tech.Rep.Dep.Mines Tasm. 20.